The present invention relates to a tracking control apparatus which is suitable for an optical disk drive, for example. More particularly, the invention relates to the tracking control apparatus which is capable of positively controlling the tracking of a target track.
Recently, there has been developed an optical or magneto-optical type recording and reproducing apparatus, which employs an optical disk or a magneto-optical disk. Such an apparatus includes a tracking actuator and an optical pickup traveling unit. The tracking actuator serves to travel an objective lens in a substantially orthogonal manner to concentric or spiral tracks in a track-following operation or a random access operation for retrieving a target track. The optical pickup traveling unit serves to travel an optical pickup having a tracking actuator in the radial direction. The track-following operation enables the actuator to be driven in accordance with a polarity or level of a tracking error signal obtained by the optical pickup so that a light beam can constantly follow the center of the track.
The random access operation, on the other hand, takes the steps of temporarily opening a tracking control loop, traveling the optical pickup to a target track with the optical pickup traveling unit, and closing the tracking control loop for enabling the optical pickup to follow a target track.
When the drawing operation is carried out for closing the tracking control loop and following the track, the control loop is often closed at a position where the light beam is slipped out of the track center, that is, the tracking error signal becomes large. In this case, the tracking actuator is accelerated toward the center of the track in accordance with the level and polarity of the tracking error signal. It results in increasing a relative speed of a target track to the light beam when the light beam reaches the track center and often slipping the light beam out of the target track too greatly to draw the light beam to the target track. In general, the control loop is closed when the light beam is impinged onto the track and the tracking error signal level is small.
Then, the description will be directed to the prior art of this type of tracking control apparatus with reference to FIG. 18.
In FIG. 18, a reading light beam 3 is reflected from an optical disk and the reflected beam is converted into a regenerative RF signal in a photoelectric converter 5. The tracking light beams 1 and 2 reflected from the optical disk are converted into electric signals S.sub.1 and S.sub.2 in photoelectric converters 9 and 10. These electric signals are applied to a differential amplifier 11 in which they are converted into a tracking error signal (a). The level of the tracking error signal (a) matches to how far the light beam is slipped out of the track center or the polarity of the tracking error signal (a) matches to which direction the light beam is oriented to. The tracking error signal (a) passes through an equalizer 12, a switch 13 and an amplifier 14 and goes to an actuator in which the signal functions as a driving signal. Thus, the actuator controls the travel of the light beam 3 to follow a track 4.
In the random access operation, a microcomputer 19 operates to reset a flip-flop 18. It results in opening the switch 13 and thus the tracking control loop. At a time, as the tracking control loop is open, the optical pickup traveling unit (not shown) moves the optical pickup to a target track. When the tracking control loop is open, the optical pickup supplies a tracking error signal (a) shown in FIG. 19(a).
The tracking error signal (a) is shaped into a pulse waveform (b) in a waveform shaping circuit 16. The pulse waveform (b) is applied to an edge detecting circuit 17 in which the waveform (b) is converted into an edge signal (c) (see the waveforms shown in FIG. 19(b) and (c)). On the other hand, the regenerative RF signal is sent to an envelope wave detector 6. The regenerative RF signal becomes larger as the light beam 3 comes closer to the center of the track 4 and smaller as it comes off the center. Hence, the envelope wave detector 6 supplies an envelope signal (d) shown in FIG. 19(d). The envelope signal (d) is shaped into a pulse waveform in a waveform shaping circuit 7. The resulting pulse waveform is an on-track signal (e) which indicates if the reading light beam 3 is spotted on the track or between tracks (see the waveform shown in FIG. 19(e)). The edge signal (c) and the on-track signal (e) are applied to an AND gate 8 from which a signal (f) is output. The signal (f) indicates the light beam 3 is spotted on the center of the track (see the waveform shown in FIG. 19(f)).
When starting the random access operation, the microcomputer 19 operates to reset the flip-flop 18, up-down count a pulse sent from the AND gate 8, and monitor where above the track the optical pickup is located. As the optical pickup comes closer to a target track, the microcomputer 19 decelerates the optical pickup. Then, when the optical pickup reaches the target track, the microcomputer 19 operates to de-reset the flip-flop 18, when the signal (f) sets the flip-flop 18. At a time, the switch 13 is turned on so that the control loop is closed, thereby drawing the light beam to the target track.
The patents relating to this type of apparatus have been disclosed in the Official Gazettes of Japanese Patent Laid-open Nos. 58-57640 and 56-58141.
The drawing operation of the prior art is designed to close the tracking control loop when the reading light beam 3 is spotted on the track as shown in FIG. 20A. However, though the relative position of the light beam 3 to the track X.sub.0 is zero, in actual, some factors such as eccentricity of an optical disk cause a certain degree of relative speed V.sub.0 to occur. If the tracking control loop is closed as keeping the relative speed, the allowable range of the relative speed V.sub.0 for positively drawing the light beam to the target track is within a drawing speed defined by a band range on which the tracking control is implemented. If the relative speed V.sub.0 is larger than the drawing speed defined by that band range, it is unstable to draw the light beam to the target track. It results in often bringing about a phenomenon that the light beam is drawn to a spot slipped out of the target track.
Next, this shortcoming will be detailed as taking an example of the random access operation.
For the purpose of traveling the light beam, the random access operation takes a step of traveling the optical pickup orthogonally to the tracks with the optical pickup traveling unit. To speed up the access operation, it is necessary to generate a large acceleration with the optical pickup traveling unit, resulting in greatly accelerating the objective lens. In case the objective lens is greatly accelerated, the reaction against the acceleration or deceleration changes the position of the objective lens, because the objective lens is normally supported by a spring. In particular, in decelerating the objective lens, the reaction displaces the objective lens so that the objective lens is reversely accelerated by the restoring force of the spring, thereby increasing the relative speed of the light beam to the target track.
The tracking error signal, in general, is a sinuous waveform as shown in FIG. 20(B). The sinuous waveform corresponds to the relative position of the light beam to the target track. That is, when the relative position of the reading light beam 3 against the target track travels past 1/4 of a track pitch P (time t.sub.2), the error signal level is decreased. When the relative position further travels past P/2 (time t.sub.3), the error signal polarity is changed so that the force exerted onto the actuator is oriented in an opposite direction to the target track. FIG. 20(A) shows the position of the light beam 3 at each time t.sub.1 to t.sub.5.
When the optical pickup is located above the target track, if the relative speed is large as mentioned above, it is often impossible to draw the light beam 3 to the track by closing the tracking control loop, because the relative position is within P/2 and thus the relative speed is not reduced to zero. In such a case, the light beam is jumped over the target track and drawn to another track. This shortcoming results from the fact that the tracking error signal is such a sinous wave as including a lot of stable points.